In view of their specific properties which open the way for many potential industrial applications, crystalline metal oxides, especially in the form of nanoparticles, have for many years been the subject of intensive research. The document “Ceramics” Abbas Khaleel and Ryan M. Richards, Nanoscale Materials in Chemistry, Kenneth J. Klabunde, 2001, John Wiley and Sons, pp 85-120, depicts a prior art of the various methods hitherto envisaged for the preparation of nanoparticulate crystalline metal oxides, namely:                physical and/or aerosol methods (vapour or gas condensation; jet pyrolysis; thermochemical or flame decomposition; thermal evaporation; vaporization in vacuo; laser evaporation); these physical methods generally produce low yields, may generate undesirable products, require elevated temperatures and complex and costly apparatus;        chemical methods:                    sol-gel: this method consists in carrying out the hydrolysis of precursors of the alcoholate type in water and/or an alcohol in the presence of a catalyst (an acid or base which allows the hydrolysis of the alkoxysilane to silanol or silyl ether to be accelerated, especially a mineral acid or a mineral base such as HCl, NaOH, KOH), producing, by condensation, a gel of metal hydroxides, then, by drying of the gel, a powder, and finally, by subsequent calcination, oxides; accordingly, this method requires many successive steps including thermal treatments and generally yields only materials having a nanostructure, not well-dispersed nanoparticles;            microemulsion: although promising, this method produces low yields and requires large quantities of solvent, a biphase reaction and calcinations;            chemical synthesis at low temperature in solution, and precipitation: these methods require final separating steps in vacuo and/or calcination at high temperature and do not allow the form and size of the particles to be controlled, the particles generally being undispersed;            mechanochemical synthesis: this method does not permit the preparation of dispersed nanoparticles having uniform and predetermined forms and dimensions.                        
In addition, EP-0947245 describes a process for the preparation of metal colloids from an organometallic precursor ([Sn(N(CH3)2)2]2 for tin) dissolved in a slightly hydrated solvent, such as anisole or commercial toluene, heating the solution, which is maintained under an inert gas, to at least 130° C. in order to bring about decomposition of the precursor, and then suppressing organic by-products by at least three steps of washing with pure solvent. Nanoparticles of tin surrounded by a protective film of tin oxide are obtained. This metal colloid can be used to prepare a sensitive layer of tin oxide. To this end, a layer of metal colloid is first formed, for example by the spin-on deposition method, and is subjected to oxidation in two steps, a first step at 200° C. and a second step at 650° C., and then to annealing at 450° C. to form a layer of particles of crystalline tin oxide having a diameter of 0.02μ. In this manner there is obtained a sensitive layer of spherical agglomerated tin oxide particles.
The publications “Synthesis and characterization of monodisperse zinc and zinc-oxide nanoparticles from the organometallic precursor [Zn(C6H11)2]” F. Rataboul et al., Journal of Organometallic Chemistry 643-644 (2002) 307-312; and “New procedure towards well-dispersed nickel oxide nanoparticles of controlled size” N. Cordente et al., C. R. Acad. Sci. Paris, chimie/chemistry 4 (2001) 143-148 describe the preparation of colloids of mixed metal particles (Zn/ZnO or Ni/NiO) having a metal core and a layer of oxide, by thermal decomposition, under an inert gas, of an organometallic precursor in a manner similar to EP-0947245. The publication of F. Rataboul teaches that nanoparticles of Zn/ZnO dispersed in PVP (polyvinylpyrrolidone) can be obtained. The particles obtained in the absence of PVP, that is to say which are not dispersed, are oxidized in the air for 3 hours at 300° C. and then for 3 hours at 600° C. There is obtained a phase of nanoparticles of pure zinc oxide having a hexagonal wurtzite structure, without coalescence but not dispersed. The publication of N. Cordente et al. describes the preparation of Ni/NiO nanoparticles dispersed in PVP and indicates that preliminary tests have been carried out for the oxidation of these particles at 100° C. (below 130° C. in order to avoid decomposition of the dispersion polymer PVP) for two weeks. However, this document admits that this oxidation does not allow nanoparticles of pure NiO oxide to be obtained, even though the results obtained are considered by the authors to be promising. Nevertheless, it is found that if the oxidation treatment is sufficiently intensive to produce oxide particles, the dispersion polymer is destroyed and the oxidized particles are no longer in the dispersed state.
Accordingly, none of the known methods mentioned above permits the direct preparation of nanoparticles of pure crystalline metal oxide(s). Furthermore, none of the known methods permits the preparation of such nanoparticles in dispersed form and having homogeneous, at least substantially uniform forms and dimensions, that is to say corresponding to a unimodal distribution (distributed around a single principal mean value), and especially substantially monodisperse (that is to say with weak dispersion around the mean value).
Moreover, most of the known methods are laborious, require complex equipment and a high level of technology and/or numerous successive steps, including thermal treatment steps and separating steps (washing, purification, etc.), produce large quantities of polluting by-products or waste (especially solvents), and are not very suitable for exploitation under profitable economic conditions on an industrial scale.
In addition, none of the known methods permits the preparation of nanoparticles of crystalline metal oxide(s) which are in dispersed form and the form of which may have form anisotropy (non-spherical), for example an elongated form (rods, threads, ribbons, etc.). In some industrial applications, however, it is important to obtain nanoparticles having uniform and controlled forms and dimensions, which are in dispersed form, and which may have form anisotropy (especially an elongated form).